1. Field of the Invention
This invention relates to a method and an apparatus for plate changing in a plate cylinder of a printing press.
2. Description of the Related Art
For a web rotary press or the like, in which a plurality of printing units are arranged, various proposals have been made for an automatic plate changer which automatically mounts a printing plate (hereinafter referred to simply as a plate) at a predetermined position of a plate cylinder of each of the plural printing units, and automatically removes the plate from the plate cylinder.
For example, Patent Document 1 (Japanese Patent No. 2704558) describes a method for plate changing in a printing press having a plurality of printing units different from each other in the rotation phase of each plate cylinder, whereby even during a plate removal in one of the printing units, a plate removal operation is performed in the other printing units, thus shortening the plate changing time.
If an error occurs during the plate removal (or plate supply) operation by the automatic plate changer, the plate being removed is engaged into the plate cylinder or ink form rollers and damages a blanket or the roller, unless the plate changing procedure is promptly stopped.
Thus, Patent Document 2 (Japanese Patent Application Laid-Open No. 11-170486) describes a feature in which a sensor is provided for detecting an abnormality in the plate being removed during plate removal, and when the abnormality in the plate removal is detected by the sensor, a printing press is shut down.
In the method for plate changing described in Patent Document 1, however, the rotation phase of the plate cylinder is different among the printing units, as stated above. Therefore, in the printing units showing no abnormality in the plate removal when an abnormality in the plate removal was detected in the other printing unit, there may be cases where the plate is being removed with its end portion being detached from the plate cylinder, or the plate remains substantially unremoved with its end portion being held by the plate cylinder.
If, in this state, the plate cylinder is rotated in a normal or reverse direction to eliminate the plate of the printing unit showing the abnormality in the plate removal, the plate cylinders of all printing units are rotated in the normal or reverse direction in an interlocked manner. As a result, damage is caused to the printing plate of the other printing unit where no abnormality in the plate removal was detected during detection of the plate removal abnormality. To remove the damaged printing plate, many man-hours and much time are required. Similar problems are posed when an abnormality in the plate being supplied occurs during a plate supply operation for automatically mounting printing plates on the plate cylinders.